1. Field of the Invention
The invention relates to a manufacturing method for a photomask including a step of writing a cyclic pattern, and to a photomask. Particularly, the invention relates to a manufacturing method for a graytone mask involving a precise cyclic pattern, and to a graytone mask.
2. Description of the Related Art
In recent years, attempts have been made to cut down the number of mask sheets by using graytone masks in the field of large-sized LCD masks (as set forth in the monthly FPD Intelligence, May, 1999).
As shown in FIG. 10A, such a graytone mask has an opaque part 1, a transmission part 2 and a graytone part 3. The graytone part 3 corresponds to an area in which there is formed a opaque pattern 3a of below or equal to the resolution limit of an exposure apparatus for a large-sized LCD using the graytone mask and is designed to selectively change the thickness of a photoresist film by decreasing the light transmitted through this area so as to decrease the amount of irradiation due to the area. Normally, the opaque part 1 and the opaque pattern 3a are formed with films that are made of the same material such as chromium (Cr) or a chromium compound and have the same thickness.
The resolution limit of the exposure apparatus for the large-sized LCD using the graytone mask is about 3 μm in the case of an exposure apparatus of a stepper type and about 4 μm in the case of an exposure apparatus of a mirror projection type. Consequently, the space width of a transmission part 3b in the graytone part of FIG. 10A is set at less than 3 μm and the line width of the opaque pattern 3a of below or equal to the resolution limit of the exposure apparatus is set at less than 3 μm, for example. When the exposure apparatus for the large-sized LCD is used for light exposure, as the exposure light transmitted through the graytone part 3 as a whole is deficient in the amount of light exposure, positive photoresists are left on a substrate though the thickness of the positive photoresists exposed to light via the graytone part 3 solely decreases. More specifically, there arises a difference in solubility of resists in developing liquid between parts corresponding to the ordinary opaque part 1 and to the graytone part because of difference in the amount of light exposure and this results in, as shown in FIG. 10B, making a part 1′ corresponding to the ordinary opaque part 1 as thick as about 1.3 μm, making a part 3′ corresponding to the graytone part 3 as thick as about 0.3 μm and making a part corresponding to the transmission part 2 a part 2′ without resists, for example. A first etching of a substrate as a workpiece is carried out in the part 2′ without the resists so as to remove the resists in the thin part 3′ corresponding to the graytone part 3 by ashing and the like and by carrying out a second etching of this part, the etching process is performed with one mask instead of two masks as conventionally used in order to reduce the number of masks for use.
In a writing process to be performed by a laser beam writing system during the course of manufacture of a foregoing graytone mask, a pattern area of, e.g., systematically arranged pixels, and another pattern area of irregular configuration are written uniformly without involvement of a distinction therebetween. Specifically, as shown in FIG. 13, in relation to a cyclic pattern area 10 such as a pixel array, a writing system successively writes a pattern in writing units 12 inherent to the writing system without regard to a cyclic unit 11 of the pattern. In other words, an entire pattern belonging to the writing unit 12 inherent to the writing system is written as a single pattern. As shown in FIG. 14, a slight difference regularly arises in the width of a written line and the coordinates of the line, in accordance with the position of a laser beam with respect a direction in which the laser beam is to be scanned (i.e., a direction the beam sweeping width). In other words, variations regularly arise in the accuracy of writing unique to the writing system in the direction in which a laser beam is to be scanned. As shown in FIG. 15, pattern cyclic units 11 which are to be adjoined to each other in the scanning direction are written with different beam positions “a” and “b” provided along a beam sweeping width 13 (i.e., a scanning length or a range in which a pattern is written by means of a single scan of a beam). As a result, a line width difference arises in different positions of each pattern cyclic unit 11. In this way, there is written a pattern of deteriorated uniformity; for example, a pattern whose geometry is deteriorated. Consequently, there arises a problem of cyclic inconsistencies (e.g., visible streaks) appearing in a cyclic pattern area stemming from a slight difference in line width of a pattern or accuracy of coordinate position. Otherwise, there arises a problem of a mask which is deteriorated in terms of uniformity of the geometry of a cyclic pattern. Particularly, a fine pattern constituting a graytone section is usually a cyclic pattern, which will raise a problem of unevenness.
When attention is paid to a channel provided in each pixel of a mask for use in manufacturing, e.g., a thin-film transistor (TFT) liquid-crystal display device (LCD), as shown in FIG. 16 the beam scanning width 13 does not match the length of pixels 14 in the direction Y. The position of the beam used for writing each pixel changes from one pixel to another. Thus, pixels are not written under identical conditions, and unevenness repeatedly arises in the dimensional accuracy of a channel portion 15. For instance, if a center position 13b in a beam scanning width 13 has superior writing accuracy (i.e., dimensional accuracy) and adjacent positions 13a, 13c have deteriorated writing accuracy, a channel 15 located in the pixel 14a has superior dimensional accuracy, and a channel 15 provided in the pixel 14b has deteriorated dimensional accuracy.
In order to solve a problem which is analogous to that mentioned above and arises in a photomask used for manufacturing a liquid-crystal display, Japanese Patent Application Laid-Open No. 250197/2000 describes a technique for preventing cyclic irregularities in a pattern. According to the technique, a ratio of pattern pitch to the pitch of a writing seam is set so as to become an integral multiple such that the least common multiple between the pattern pitch and the pitch of a writing seam assumes a value of 1 mm or less.
Such a problem also applies to a graytone section consisting of a fine cyclic pattern of graytone mask. For instance, in relation to a graytone section consisting of lines and spaces, when lines are separated on a per-writing basis inherent to a writing system, a problem analogous to that mentioned above arises.
The method described in the foregoing patent application is for preventing occurrence of cyclic inconsistencies in a pattern, which would otherwise arise in a scanning direction (i.e., the Y direction). For example, in the case of a laser writing system, a beam is emitted in a direction perpendicular to a scanning direction (i.e., the X direction) as well as in the scanning direction, at a pitch of beam size (or a grid) of the laser beam unique to the system (i.e., a beam spot). At this time, a line width of a writing pattern is controlled by means of adjusting beam power.
As shown in FIG. 17, in relation to a grid corresponding to a overrun width of a laser beam (beam spot) in the X direction unique to the system, the length of a pixel in the X direction (design data) 16 is not designed in consideration of an integral number of grids 17 being housed in the pixel. Hence, a grid head 17a may become misaligned in each pixel. As shown in FIG. 18, as a result of the grid head becoming misaligned at the left end of each pixel, the positions of the grids 17 used for writing channel sections 15 in the respective pixels 14 also become misaligned.
In the laser writing system, the channels 15 of respective pixels theoretically have an identical line width even if the grids 17 have become misaligned. However, in reality, the channels 15 of the respective pixels have turned out to differ in line width from each other. Accordingly, there is a problem of use of the technique described in the publication being insufficient for drawing, e.g., a fine precision pattern of a graytone mask, with superior accuracy.
In the related-art technique, a pitch of writing seam is determined by means of controlling the head scanning width of a mask writing system and a overrun pitch of a stage. However, matching the scanning width (a beam sweeping width) with a pitch of writing seam involves a change in system configuration. Moreover, a writing seam varies from one photomask to another. Hence, in practice there arises a problem of the inability to change a scanning width.
When a photomask pattern is constituted of a cyclic pattern section consisting of a cyclic pattern and a normal pattern section consisting of another pattern, a data area pertaining to an overall mask is taken as a single set of writing data. Accordingly, the volume of data becomes enormous. Particularly, since a cyclic pattern constituting a graytone section is fine, in some cases the volume of data will become massive at the time of preparation of data. The volume of data exceeds the capacity of a writing system or that of a data converter provided along with the writing system.